chore: rename quicproquo → quicprochat in Rust workspace

Rename all crate directories, package names, binary names, proto
package/module paths, ALPN strings, env var prefixes, config filenames,
mDNS service names, and plugin ABI symbols from quicproquo/qpq to
quicprochat/qpc.

proto/qpc/v1/auth.proto

@@ -0,0 +1,43 @@
+syntax = "proto3";
+package qpc.v1;
+
+// OPAQUE registration + login (4 methods).
+// Method IDs: 100-103.
+
+message OpaqueRegisterStartRequest {
+  string username = 1;
+  bytes request = 2;
+}
+
+message OpaqueRegisterStartResponse {
+  bytes response = 1;
+}
+
+message OpaqueRegisterFinishRequest {
+  string username = 1;
+  bytes upload = 2;
+  bytes identity_key = 3;
+}
+
+message OpaqueRegisterFinishResponse {
+  bool success = 1;
+}
+
+message OpaqueLoginStartRequest {
+  string username = 1;
+  bytes request = 2;
+}
+
+message OpaqueLoginStartResponse {
+  bytes response = 1;
+}
+
+message OpaqueLoginFinishRequest {
+  string username = 1;
+  bytes finalization = 2;
+  bytes identity_key = 3;
+}
+
+message OpaqueLoginFinishResponse {
+  bytes session_token = 1;
+}